Objectives: The aim of this study is to describe the technique, develop a formula to define its limits of accuracy, and report on its clinical application.
Methods: Using the positioning laser grid lines on the computed tomography scanner, a small metallic marker is placed on the scalp at the intended biopsy site. The distance between the tumor and the inner table of the bone is measured. In the operating theater, the patient is positioned such that the tip and the bridge of the nose are aligned in a horizontal position. The patient's two eyebrows or the orbital canthi are aligned in a vertical line. Simultaneously, the posterior borders of the two pinnae are aligned vertically. Gravity is used to guide the biopsy needle through the marked burr hole into the target. Seven patients had biopsies. One was for targeting the craniopharyngioma cyst to place an Ommaya catheter. The fraction of error or error fraction (EF), was developed for verification of its limits of accuracy.
Results: All the biopsies were diagnostic and the Ommaya catheter was correctly sited. The EFs at α = 5° were all predictive of the limits of accuracy of this technique.
Conclusion: This is the first reported gravity-guided stereotactic brain surgery. The outcome in all the eight cases showed that it was within the limits of its accuracy. EF can be calculated to ensure accuracy. This technique is helpful if a commercial stereotactic system is not available.
CASE SUMMARY: A 27-year-old male, who was exposed to cranial irradiation at the age of 4 years as part of the treatment for acute lymphoblastic leukaemia, presented with symptoms of raised intracranial pressure for one week. Brain magnetic resonance imaging revealed a heterogeneously enhancing lesion at the hypothalamus. Stereotactic biopsy was performed. Histopathological examination of the lesion showed malignant rhabdoid tumour. The disease progressed rapidly, with manifestation of leptomeningeal spread. He was started on craniospinal irradiation but treatment was suspended after 5.4 Gy, as he developed myelosuppression. His clinical condition deteriorated rapidly, and he succumbed to his illness within 2 mo.
CONCLUSION: This fifth case of radiation-induced central nervous system rhabdoid tumour re-enforces the aggressive nature of this disease with poor prognosis.